1. Field of the Invention
This invention relates to a thin-film magnetic head in which a magnetic circuit constituted by thin films is formed on a substrate and, more particularly, to a thin-film magnetic head having a substrate formed of crystallized glass.
2. Description of the Related Art
Thin-film magnetic heads are manufactured by using the thin-film forming technique and the photo-lithographic etching method in such a manner that a magnetic core constituted by magnetic films and a coil constituted by conductive films are formed on a substrate with insulation layers interposed between the magnetic films and the conductive films, and a protective layer or plate is provided on these layers. If a thin-film magnetic head of this construction is used, an extremely small track width can be set. The practical application of this kind of magnetic head is, therefore, expected with the recent development of high-density magnetic recording systems. Some thin-film magnetic heads of this kind have already been developed as articles of commerce to be used in external storage units for computers or to be used as heads for still video apparatus.
Conventionally, ferrite and alumina have been used as materials for forming substrates for thin-film magnetic heads. Ferrite has improved workability and good characteristics in terms of sliding on the recording medium and is superior in durability. However, since the thermal expansion coefficient of ferrite is generally smaller than that of the magnetic material of magnetic thin films constituting the magnetic core of the thin-film magnetic head, there is a risk of separation of the magnetic films of the magnetic core owing to an increase in film stress, if the thickness of the magnetic films is several microns or larger. In addition, because it is difficult to form a large-diameter substrate, the producibility of the substrate is low. There is also a problem in terms of characteristics, i.e., occurrence of large sliding noise.
The use of alumina entails a problem of its very large hardness, that is, it is inferior in workability.
In recent years, crystallized glass has been noted as a material for the substrate. Crystallized glass can be freely changed in thermal expansion coefficient, hardness, crystal grain size and so on and, therefore, has a wide range of application. Specifically, the influence of the crystal grain size on the photo-lithographic etching working accuracy and on the occurrence of chipping or break during mounting process of the head is large. In consideration of this, a type of crystallized glass is widely used recently which consists of an aggregation of microcrystalline grains having very small grain size (about 0.2 to 0.5 .mu.m), as shown in a schematic illustration of an enlarged photography shown in FIG. 5.
However, if a magnetic recording medium contacts and slides on a thin-film magnetic head having a substrate formed of such crystallized glass for a long time, the microcrystalline grains of the substrate are chipped off by sliding friction between the head and the recording medium, and the substrate is worn fast. That is, the abrasion resistance of the head is small. Another problem is encountered which resides in that chipped-off microcrystalline grains are attached to the surface of the magnetic head sliding on the recording medium to cause spacing losses, resulting in deterioration in recording/reproduction characteristics of the head.